Priority is claimed under 35 U.S.C. xc2xa7119 with respect to German Patent Application No. 199 11 019.0-52 filed on Mar. 12, 1999.
The invention relates to a method for determining the air/fuel ratio in an internal combustion engine combustion chamber having at least two ignition plugs spaced apart from one another, as used, for example, for motor vehicles.
German preliminary published application DE 25 54 988 A1 has disclosed the arrangement of an ion-current sensor in the exhaust system downstream of the exhaust valves of a combustion chamber of an internal combustion engine. With the aid of the ion-current sensor positioned in this way, measurement of the ion current of the combustion products is performed and a controlled variable is generated for a mixture-forming device and an ignition device. Mixture formation and ignition setting are then controlled as a function of the operating state of the internal combustion engine.
German Patent DE 42 39 803 C2 has disclosed an ionization-current detector for an internal combustion engine in which an ignition plug in a combustion chamber of the internal combustion engine is used both to initiate combustion and to detect complete combustion of the combustion-chamber charge and thus to detect misfires.
German Patent DE 196 14 388 C1 has disclosed a method for determining the quality of an air/fuel mixture in a combustion chamber of an internal combustion engine. Here a test pulse is applied to an ignition plug, which is initially used to generate a customary ignition pulse, after the ignition of the air/fuel mixture. The test pulse comprises a voltage pulse which remains constant for a prolonged period during the combustion phase to be investigated and is applied to the ignition plug via a measuring resistor. At the measuring resistor it is possible to measure avoltage drop which changes with the ion current at the ignition plug and the integral of which is used as a measure of the composition of the mixture in the combustion chamber.
German preliminary published application DE 29 39 690 A1 has disclosed a method for controlling the ignition point for an internal combustion engine. In this method, an ion-current sensor is arranged at a distance from an ignition point in a combustion chamber of the internal combustion engine. The ion-current sensor is used to monitor the ignition of the combustion-chamber charge, the end of an initial ignition phase of the combustion-chamber charge being kept in a particular relationship with top dead centre. To carry out the method, one or two ion-current sensors are arranged around the ignition electrodes of the ignition plug, the ion-current sensors being combined with the ignition plug.
German Patent DE 34 45 539 C2 has disclosed an ionization sensor for controlling the combustion process in a combustion chamber of an internal combustion engine. The ionization sensor is arranged in the combustion chamber at a distance from the location of ignition, allowing it to detect the arrival of a flame front, and the quality of the combustion process which takes place is determined from the time which elapses between ignition of the air/fuel mixture in the combustion chamber and the arrival of the flame front at the sensor.
German preliminary published application DE 29 35 725 A1 has disclosed an apparatus for adjusting the ignition point of an internal combustion engine with applied ignition. It comprises a first ionization sensor at a relatively large distance from an ignition plug and optionally a second ionization sensor close to the ignition plug. In particular, the second ionization sensor is used to detect the beginning of combustion and the first ionization sensor is used to detect the end of combustion. The ignition point of the internal combustion engine is adjusted as a function of the signals detected.
German preliminary published application DE 29 32 193 A1 has disclosed an apparatus for detecting knock phenomena in a combustion chamber of an internal combustion engine with feedback-controlled ignition. It comprises two ionization sensors, which are arranged in the combustion chamber as far as possible away from an ignition plug for igniting the air/fuel mixture, in a region in which knock phenomena are possible. The two ionization sensors are arranged in the direction of progress of the flame front in the combustion chamber relative to one another and are used to determine the speed of the flame, from which, in turn, conclusions can be drawn about knock phenomena.
It is the object of the invention to determine the air/fuel ratio in a combustion chamber of an internal combustion engine in as simple a manner as possible and as reliably as possible with a method of the type stated at the outset.
According to the invention, this object is achieved by a method in which a measurement of the speed of the flame in the air/fuel mixture in the combustion chamber is performed between two ignition plugs arranged in a common combustion chamber of the internal combustion engine. A first ignition plug, which is operated as an ion-current sensor, is used to ascertain the time of arrival of the flame front at this ignition plug, while a second ignition plug is used to release ignition energy and to ascertain the ignition point. The air/fuel ratio is determined from the speed of the flame, taking into account the operating point of the internal combustion engine. The ignition plugs are preferably used alternately to release ignition energy and as ion-current sensors. The method proposed can be used to determine the air/fuel ratio individually in each combustion chamber of the internal combustion engine, allowing fuel metering to be performed individually for each combustion chamber and an oxygen sensor in the exhaust duct to be dispensed with.
As a refinement of the invention, in an operating cycle with ion-current measurement, the ignition point of the second ignition plug is advanced in comparison with operating cycles with release of ignition energy and at both ignition plugs and hence without ion-current measurement. Here, release of ignition energy at both ignition plugs is provided, in particular, in full-load operation of the internal combustion engine, and release of ignition energy at just one ignition plug is preferably provided in part-load operation. Advancing the ignition point takes account of the possibility that combustion in the combustion chamber may take place more slowly initially, and displacement of the main focus of combustion towards the rear is avoided.
As a further refinement of the invention, ion-current measurement is performed in each operating cycle in the part-load range of the internal combustion engine, the ignition plugs being operated alternately as ion-current sensors. According to this, each ignition plug functions as an ion-current sensor and does not release ignition energy in one operating cycle and, in the following operating cycle, it functions as an ignition plug which releases ignition energy. This prevents the formation of deposits such as those which can form on an electrode which is used exclusively as an ion-current sensor. It is also possible to make the stressing of the combustion-chamber walls more uniform.
As a further refinement of the invention, ion-current measurement is formed in only some of the operating cycles in the part-load range of the internal combustion engine. Here, the same ignition plug can be operated as an ion-current sensor every time, making evaluation simpler.